Chemically-induced plant defense responses were investigated in tobacco cell cultures. The inducing conditions were as follows: chitosan (C), an elicitor (E) prepared from Phytophthora nicotianae, isonicotinic acid (INA), isonicotinamide (IND) and isonitrosoacetophenone (INAP) as well as the addition of INA, IND and INAP as conditioning agents (primary elicitors) followed by secondary elicitation with either chitosan or elicitor. The defense responses investigated included determinations of phenylalanine ammonia-lyase (PAL) activity, total soluble phenolic content, specific phenolic profiles, phytoalexin content, (3- 1,3-glucanase activity and electrophoretic analyses of pathogenesis-related proteins (PR). The compounds, 4-(3-methyl-2-butenoXy)isonitrosoacetophenone (0-INAP) and 2-isonitrosoacetophenone (INAP) were successfully synthesized from the starting materials p-hydroxyacetophenone and acetophenone respectively. The organic synthesis of 0-INAP involved the formation of a prenyl ether.of p-hydroxyacetophenone, followed by a nitrosation reaction using butyl nitrite as the source of the nitroso group, on the a-carbon atom adjacent to the carbonyl group. The synthesis of INAP only required a nitrosation reaction on the a-carbon atom adjacent to the carbonyl group. The yields of 0-INAP and INAP were 12 - 15 % and 80 %, respectively. An evaluation of the properties of 0-INAP indicated that the compound, dissolved in methanol, has a molar extinction coefficient of 16 5001.mor.cm - ' at A. 302 nm. The compound possesses antifungal activity against Cladosporium cucumerinum, Penicillium expansum and Aspergillus niger as well as the ability to scavenge superoxide radicals which was indicated by a decrease in the chemiluminescence signal produced in a reaction mbdure of hydrogen peroxide, horseradish peroxidase, the chemiluminescence probe, MCLA, and increasing concentrations of 0-INAP. The addition of INA to tobacco cells at a - final concentration of 12.5 iimol.g -1 cells or 2.5 mM did not lead to significant changes in PAL activity, but conditioning with INA, followed by chitosan as well as elicitor led to a 2.5-fold and a 4.3-fold induction respectively. INA as well as INA + C and INA + E led to significant increases in the total soluble phenolic content, and the HPLC analyses of these phenolics indicated the significant induction of a phenolic-like compound with a peak at Rt = 1.7 min. which possibly indicates isonicotinic acid, for INA + C and INA + E. A whole range of phytoalexins were detectable after the addition of INA to tobacco cells and conditioning with INA followed by chitosan induced the phytoalexin, lubimin, several hundred-fold. PR proteins were also induced by INA and a prominent band of 11- 13 kDa was induced after conditioning with INA, followed by secondary elicitation with the elicitor and especially with chitosan. (3-1,3-glucanase activity was also induced by INA; INA + E and particularly INA + C led to increases of 2.5-fold and 4.5-fold in 13-1,3-glucanase activity respectively. The addition of IND to tobacco cells at a final concentration of 12.5 pmol.g -1 cells or 2.5 mM led to a 2.6-fold induction in PAL activity after only 6 h, but conditioning with IND, followed by secondary elicitation did not lead to any significant changes. IND at the earlier time interval (24 h vs. 48 h) as well as IND + C and IND + E led to increases in the total soluble phenolic content, - and the HPLC analyses of these phenolics indicated the significant induction of a phenolic-like compound with ,a peak at Rt = 1.7 min. which possibly indicates isonicotinic acid, for IND + C and IND + E. A whole range of phytoalexins were detectable after the addition of IND to tobacco cells and conditioning with IND followed by chitosan induced the phytoalexin, solavetinone, several hundred-fold. PR proteins were also induced by IND and prominent bands of 34 kDa and 39 - 40 kDa were induced for IND + ELIC. (3-1,3-glucanase activity .was also induced by IND; however, secondary elicitation with chitosan did not lead to increases in enzyme activity, although a twofold increase was detectable for IND + ELIC, compared to IND 72. The addition of INAP to tobacco cells at a final concentration of 6.3 pmol.e cells or 1.25 mM led to a 1.7-fold induction in PAL activity after only 6 h, a response that was still detectable after 30 h; however, conditioning with INAP, followed by secondary elicitation did not lead to any noteworthy changes. INAP 24 as well as INAP 48 did not lead to significant changesin the total soluble phenolic content, but INAP + C and INAP+ E led to increases of 3.3-fold and 3.5-fold, respectively. HPLC analyses of the induced phenolics showed the significant induCtion of a phenolic compound with a peak at Rt = 14.5 min. which possibly indicate p-coumaric acid, for INAP + C and INAP + E. A whole range.of phytoalexins were detectable after the addition of INAP to tobacco cells, but the addition of a secondary elicitor led to a decrease in phytoalexin accumulation. PR proteins were also induced by INAP and conditioning with INAP, followed by especially the elicitor, led to the induction of a whole range of PR proteins with molecular masses ranging from 11 - 68 kDa. (3-1,3-glucanase activity was significantly induced (60-fold compared to control) by INAP 48; however, secondary elicitation led to a decrease in (3-1,3-glucanase)